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«Microwave in chemical syntheses»

Fundamentals

Closed-vessel microwave heating techniques have been the state of the art for sample preparation  in the analytical laboratory for over fifteen years. However, the application of microwaves in the organic synthesis community is only now beginning to receive widespread attention.

The first papers on the use of microwaves for synthesis reactions appeared in the open, peer-reviewed literature in 1986. Since that time, over a thousand articles have been published, numerous conferences have focused on the advance of microwave techniques, and the use of microwave processing is now the hot topic for combinatorial and parallel strategies.

Two forces are cultivating the current interest in microwaves for synthesis. First, technical advances derived from many years' experience with hardware, software, and reaction vessel design have produced microwave labstations with the performance and flexibility to meet the needs of organic chemistry. Second, the open literature is mature enough to demonstrate clearly just how effective microwaves can be at enhancing ynthetic reactions.

Microwave enhancement can take several forms. Reaction rates can be accelerated, yields can be improved, and reaction pathways can be selectively activated or suppressed. Fundamentally, microwaves heat things differently than conventional means.

Microwaves Are Energy

Microwaves are a form of electromagnetic energy. Microwaves, like all electromagnetic radiation, have an electrical component as well as a magnetic component. The microwave portion of the electromagnetic spectrum is characterized by wavelengths between 1 mm and 1 m, and corresponds to frequencies between 100 and 5,000 MHz. Milestone microwave labstations use a specific, fixed frequency of 2,450 MHz (2.45 GHz).

It is useful to consider the quantum energy of microwaves in relationto other forms of electromagnetic energy. It is important to recognize that the energy delivered by microwaves is insufficient for breaking covalent chemical bonds. This information can help to narrow speculation on the mechanisms for enhancement in specific reactions.

Microwaves Can Interact with Matter

One can broadly characterize how bulk materials behave in a microwave field. Materials can absorb the energy, they can reflect the energy, or they can simply pass the energy. It should be noted that few materials are either pure absorbers, pure reflectors, or completely transparent to microwaves. The chemical composition of the material, as well as the physical size and shape, will affect how it behaves in a microwave field.

Microwave interaction with matter is characterized by a penetration depth. That is, microwaves can penetrate only a certain distance into a bulk material. Not only is the penetration depth a function of the material composition, it is a function of the frequency of the microwaves. It is not true that microwaves "heat" a bulk material "from the inside out."

Two Principal Mechanisms for Interaction With Matter

There are two specific mechanisms of interaction between materials and microwaves: (1) dipole interactions and (2) ionic conduction. Both mechanisms require effective coupling between components of the target material and the rapidly oscillating electrical field of the microwaves.

Dipole interactions occur with polar molecules. The polar ends of a molecule tend to align themselves and oscillate in step with the oscillating electrical field of the microwaves. Collisions and friction between the moving molecules result in heating. Broadly, the more polar a molecule, the more effectively it will couple with (and be influenced by) the microwave field.

Ionic conduction is only minimally different from dipole interactions. Obviously, ions in solution do not have a dipole moment. They are charged species that are distributed and can couple with the oscillating electrical field of the microwaves. The effectiveness or rate of microwave heating of an ionic solution is a function of the concentration of ions in solution.

Materials have physical properties that can be measured and used to predict their behavior in a microwave field. One calculated parameter is the dissipation factor, often called the loss tangent. The dissipation factor is a ratio of the dielectric loss (loss factor) to the dielectric constant. Taken one more step, the dielectric loss is a measure of how well a material absorbs the electromagnetic energy to which it is exposed, while the dielectric constant is a measure of the polarizability of a material, essentially how strongly it resists the movement of either polar molecules or ionic species in the material. Both the dielectric loss and the dielectric constant are measurable properties.

Microwave Heating Differs from Conventional Means

Conventional Heating Methods

In all conventional means for heating reaction mixtures, heating proceeds from a surface, usually the inside surface of the reaction vessel. Whether one uses a heating mantle, oil bath, steam bath, or even an immersion heater, the mixture must be in physical contact with a surface that is at a higher temperature than the rest of the mixture.

In conventional heating, energy is transferred from a surface, to the bulk mixture, and eventually to the reacting species. The energy can either make the reaction thermodynamically allowed or it can increase the reaction kinetics.

In conventional heating, spontaneous mixing of the reaction mixture may occur through convection, or mechanical means (stirring) can be employed to homogeneously distribute the reactants and temperature throughout the reaction vessel. Equilibrium temperature conditions can be established and maintained.

Although it is an obvious point, it should be noted here that in all conventional heating of open reaction vessels, the highest temperature that can be achieved is limited by the boiling point of the particular mixture. ............